Is it just a silly pun thing where since onee-san said literally is 'when you get bigger' and she wasn't sure if onee-san means height or size and so went for them both before confessing
At this point, it would've been more subversive to make the boyish kid turn into a handsome guy rather than a hot babe. I'm pretty sure I've seen the latter trope the better part of 100 times by now.